I didn't find Will Smith to be the right fit for Ali at all. And I'm 27, which means I grew up with the Fresh Prince and all his movies - I really do respect and like the guy.
But he was not believable. His manner of talking was forced throughout the whole movie, he couldn't speak with the same rhythm and fluidity that Ali spoke with naturally.
And, unfortunately, when you're a real, true boxing fan, so much of a fighter comes out when they're in the ring - and Ali's style was impossible to replicate for the camera. Only he could do it. Although I must say the movie did a great job of staying "true" to the punches that were thrown and the rounds and all that.
The Micky Ward movie was the same way - Wahlberg did well as Mick throughout the film, but when it came to the fight scenes, well, Wahlberg is a trained boxer now - and he fights in a totally different way than Mick, with a different style, with his hands held differently and different movement.... the style a guy cracks with is just one of those things that really becomes unique to him, and watching movies try to replicate it is tough.
That's why Rocky did so well - they weren't TRYING to replicate anyone or anything exactly, but it ended up that Sly came off more like Marciano and Weathers came off more like Ali than anyone I have seen ever.